Journey from Gene to Brain in 25,000 Genes or Less, The

Phi Kappa Phi Forum, Winter 2005 by Marcus, Gary

Are human beings genomically challenged? In the space of a less than half a decade, scientists' best estimates of the numbers of genes in the human genome has dropped dramatically, from about one-hundred thousand a few years ago to just twenty-five thousand, according to a report published in October of 2004. At this point, human beings seem to have scarcely more genes than a mustard seed. Stanford biologist Paul Ehrlich has suggested that there is a "gene shortage," and the San Francisco Chronicle has gone so far as to suggest that "DNA's significance" should be "downplayed."

But by itself, the sheer number of genes tells us little about the complexity of a species's mental life. Just as we would not try to guess whether a human is more complex than a chimpanzee merely by counting the number of bones each species had, we should not expect that the number of genes would necessarily be more informative.

That said, it is hard not to notice the gap between that comparatively small number of genes and the enormous number of neurons - more than twenty billion - in our brain. Where does the enormous complexity of the human brain come from?

IT IS NOT ALL EXPERIENCE

It is tempting to put the burden of neural organization almost entirely on learning and experience. Virtually any skilled human behavior, from playing the piano to dunking a basketball, depends not just on talent but on years of experience - experience that rewires and fine-tunes your brain's organization. Even if babies are "a lot smarter than you think," as a recent Life magazine cover suggested, and human children are born with a remarkable faculty for acquiring human language (as Noam Chomsky and Steven Pinker have suggested), nobody could doubt that practice is still the best way to get to Carnegie Hall.

At the same time, as important as learning, training, and education are, it would be an enormous mistake to simply ignore the role that genes play. For example, on just about any trait you could measure, identical twins (who share all their genes) are more similar than nonidentical twins (who share only half their genes); their brains, too, are more similar than those of nonidenticals. Just as identical twins are more likely to look alike, they are more likely to think alike. Genes clearly play a critical role in shaping the human condition.

Another measure of the extent to which genes influence the development of neural structure comes from sheer neuroanatomy. In keeping with the notion that we are born with significant mental structure (à la Chomsky, Pinker, and the dozens of recent psychological studies revealing the precocious abilities of human infants), the vast majority of the brain's structural details are in place by birth. The brain of an infant is only about a quarter the size of an adult's, but from the cerebral asymmetries to the complex patterns of bends and folds known as gyri and sulci, virtually every significant landmark is already there. To be sure, prenatal experience plays a role - the womb is dark, but not silent, and late-term fetuses can learn - but the complexity of a newborn human's brain is manifest testimony to the enormous influence of our genomic constitution.

How can twenty-five thousand genes contribute so much? To understand the answer, we need to start by first abandoning two longstanding (and as it turns out somewhat contradictory) habits of thought. One habit is to think of the genome as a blueprint; another is to think of genes as dictating particular traits. The blueprint metaphor suggests (wrongly) that each gene would be like an individual pixel in a picture, as if the genome were a literal depiction of a particular species' structure. The trait metaphor (think "The God Gene," to take the title of a recent book) suggests that there is a simple mapping between genes and traits, as if human traits were as simple as the color and texture of Gregor Mendel's peas. But there is no simple mapping between genes and traits; a single trait may be the product of many genes, and a single gene can influence many traits. Moreover, most genes are shared across all humans and have little if anything to do with the relatively tiny differences between us.

If one wants to understand the contribution of genes to the human mind, one needs to move past these easy metaphors. As the British biologist Patrick Bateson put it, "The idea that genes might be likened to the blueprint of a building... is hopelessly misleading because the correspondences between plan and product are not to be found. In a blueprint, the mapping works both ways. Starting from a finished house, the room can be found on the blueprint, just as the room's position is determined by the blueprint. This straightforward mapping is not true for genes and behavior, in either direction."

IF-THEN

But if genes are neither blueprints nor simple controllers of traits, what are they? Borrowing a notion from computer programming, I would suggest that a better metaphor is the idea of a conditional: an IF paired with a THEN. Just as virtually all computer software is largely made up of IF-THENs ("IF the amount of the withdrawal exceeds the customer's balance, THEN reject the request."), the information contained in the genome, too, consists largely of logical preconditions paired with consequences.